1. Technical Field
The present disclosure relates to a positive electrode and a lithium ion secondary battery.
2. Description of the Related Art
Recent years have seen increasing expectations for widespread use of various electric vehicles, with a view to solving environmental and energy problems. A key to practical application of electric vehicles is a vehicle-mounted power supply, such as a motor-driving power supply. As such, lithium-ion secondary batteries are being intensively developed. In order for the battery to be widely adopted as a vehicle-mounted power supply, it is very important that the battery have high thermal stability.
A number of positive electrode active materials for lithium ion secondary battery have been proposed. One of the most typical examples is a lithium-containing transition metal oxide having a laminar structure, such as lithium cobalt oxide (LiCoO2) and lithium nickel oxide (LiNiO2). Lithium cobalt oxide (LiCoO2) and lithium nickel oxide (LiNiO2) have high capacity, and their discharge voltages with respect to lithium are also high at approximately 3.8 V. However, these oxides, when placed in high temperature state or high potential state, readily react with the electrolytic solution and release the oxygen in the crystal structure. Accordingly, thermal stability is not sufficient, particularly in highly charged state.
In this regard, JP-A-2008-277152 describes coating at least a part of the surface of the lithium-containing transition metal oxide core particles with a coating portion including LiVOPO4, in order to increase thermal stability.